1. Field of the Invention
This invention relates to the manufacture of non-woven webs, the non-woven webs themselves, and their use in areas where soft hand, high tensile strength and flexibility are critical such as, for example, in the field of diaper coverstock.
2. Description of the Prior Art
Non-woven fabrics are conventionally manufactured by producing a web of loosely associated textile fibers disposed in sheet form, using any one of a variety of well known procedures, and then subjecting the web to a bonding operation to anchor or bond the individual fibers together. The conventional base material for non-woven fabrics is a web comprising any of the common textile fibers, or mixtures thereof. The web generally has a carded fiber structure or comprises fibrous mats in which the fibers or filaments are distributed haphazardly or in random array.
Dry laid non-woven webs may be made by carding, air-laid, spunbonded, or spunlaced procedures and then the fibers may be subsequently fixed by chemical, mechanical or thermal means. With respect to the thermal bonding procedure the fibers themselves can act as natural binders; a lower melting-point fiber is incorporated in the fiber blend, then the web is subjected to a high enough temperature to cause the lower melting point fiber to soften and bind to the fiber with the higher melting point. These thermally bonded non-wovens are assuming an ever increasing role in the market place today. Apparently, thermally bonded fibers give more comfort and more "textile-like" hand. We have developed a procedure that will produce chemically bonded non-woven with at least as good a hand and tensile strength as the thermally bonded fibers, if not superior. Polypropylene has been the fiber of choice as a thermoplastic fiber to be thermally bonded. Its particular potential has been discussed for use in the diaper coverstock industry. We have, however, produced a polyester product that is comparable, if not superior, to any product now available.
The bonding operation can be accomplished in any one of several ways such as by spray bonding, saturation bonding or print-bonding. One method is to impregnate the web continuously over its entire surface with various well known bonding agents. Such a method of impregnation is referred to as saturation bonding. This method produces a product of good strength; however, it tends to be stiff and boardlike. In order to alleviate this problem it is necessary to use a binder that is relatively soft. For example, saturation bonding is widely used in the production of diaper coverstock and a very soft binder needs to be employed. This raises yet another problem in that using a softer binder tends to produce a product with lower tensile strength.
Another bonding method is to print non-woven webs with continuous or wavy lines of binder extending traversely across the web so that the binder is applied only at localized areas which often defines a pattern on the web. This type of bonding method is used where it does not matter how little tensile strength is achieved. This method results in webs having softness and hand more nearly approaching that of a textile fabric. The problem here, however, is that such a method produces a product that lacks sufficient tensile strength for the many uses, e.g., diaper coverstock. In fact, nowhere in the prior art is such a print bonding method employed in the manufacture of diaper coverstock or the like.
From the foregoing analysis it will be seen that none of the non-woven fabrics of the prior art has been entirely satisfactory in producing a product that on the one hand has soft hand but yet, on the other, demonstrates superior wet tensile strength.